During use of a video camera or a digital still camera, auto white balance adjustment is performed in order to reproduce a white object as white. In a well-known, conventional auto balance method, the balance of RGB components (three primary color components of red, green, and blue) of signals for each pixel is adjusted so that the average of the whole image becomes achromatic. However, this method has a disadvantage in that white balance adjustment tends to be performed incorrectly when chromatic color occupies the greater portion of an image.
Such incorrect white balance adjustment is referred to as color failure. The technique disclosed in Japanese Application Hei 5-292533 is known as an auto white balance adjusting method for reducing color failure. In the case of this technique, an image is divided into a plurality of blocks, the average value of RGB of each block is calculated, and only a block in which the average value falls within a predetermined range is extracted. Then, each component of RGB is adjusted so that the average value of RGB of the extracted block group becomes achromatic.
Moreover, Japanese Application Hei 5-7369 discloses another auto white balance method for reducing color failure. In this method, the range of values which can be assumed by a white balance adjustment signal is restricted, so as to avoid performance of excessive white balance adjustment.
Although these methods yield advantages when a light source for illuminating an object is restricted, sufficient white balance adjustment is impossible when an object is illuminated by an unexpected light source or a plurality of types of light sources simultaneously.
Therefore, in Japanese Application. Hei 8-289314 the present applicant has proposed an improved auto white balance adjusting apparatus. This apparatus extracts a block group determined to be the result of dividing an image into a plurality of blocks and photographing a white object under a fluorescent lamp, a block group determined to be the result of photographing a white object under sunlight or tungsten light, and a block group having a color close to the brightest block in an image. Then, average values of RGB components of these block groups are obtained, and a value obtained by mixing the average values in accordance with a predetermined rule is used as a white balance adjustment signal. This apparatus can perform proper white balance adjustment even in the case where the an object is illuminated by an unexpected light source or a plurality of types of light sources, by reflecting in a white balance signal the average value of RGB of a block group having a color approximating that of the brightest block in an image.
However, in the case of the method disclosed in Patent Document 3, because the above block groups are extracted in accordance with only color, the value of a dark block is used when obtaining the average values of each block group. A color appearing as an image is influenced by the color of an object and the color of a light source for illuminating the object. In this case, because the dark block has a high probability of not reflecting the color of the light source, reflecting the information of the dark block on an auto white balance adjustment signal may lead to an error.
Therefore, in Japanese Application 2000-92509 the present applicant has proposed a further-improved auto white balance adjusting apparatus. In this apparatus, the average value of RGB components obtained for each block obtained by dividing an image is used as the typical value of blocks; a block having the highest luminance is obtained in accordance with the typical value; and a luminance threshold value is obtained in accordance with the luminance of the highest-luminance block. Then, white balance adjustment is performed in accordance with the typical value of blocks each having luminance higher than the luminance threshold value among all blocks.